A lack of precision is not the problem. The problem is that at some point the game logic makes a decision such as whether a bullet hit a target, whether a playe

A lack of precision is not the problem. The problem is that at some point the game logic makes a decision such as whether a bullet hit a target, whether a player made a jump, or many others. These decisions sometimes end up right on a cusp, where a single unit in the last place determines the behavior. In a sensitive physics engine these happen more often than you might think. And when the result changes, all future events are impacted. This is crucial for replays, it may be crucial for multiplayer, and it can result in unexpected behavior in ways that you only find when writing an emulator. A slight change in floating-point math may also result in a slightly larger-than-expected number which may then result in a slightly negative number which may result in a NaN (sqrt(-1e39) perhaps) which then results in the poisoning of all future math that uses that number.